Interventional instrument tracking device imageable with magnetic resonance imaging and method for use thereof

ABSTRACT

A tracking device configured to be coupled to an interventional instrument and tracked by a magnetic resonance imaging system is provided. The tracking device includes, for example, paramagnetic and diamagnetic components that form first and second tracking members. When the tracking device is adjusted into a first arrangement, the tracking device will produce a local magnetic field in the presence of the magnetic field of an MRI system that is measurable by the MRI system. However, when the tracking device is adjusted into a second arrangement, the local magnetic field produced by the tracking device is reduced relative to the first arrangement, wherein the reduced local magnetic field produces substantially no magnetic field disturbances detectable by the MRI system. Images may be acquired of a patient in which the tracking device has been introduced and, using a numerical fitting method, an accurate location of the tracking device can be determined.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application represents the U.S. National Stage of InternationalApplication No. PCT/CA2010/002041, filed Dec. 22, 2010, which claims thebenefit of U.S. Provisional Patent Application Ser. No. 61/289,271 filedon Dec. 22, 2009, and entitled “System and Method for Interventional MRIDevice Tracking with Mechanically Controlled Susceptibility Effects.”The foregoing applications are incorporated herein by reference in theirentirety.

BACKGROUND OF THE INVENTION

The field of the invention is magnetic resonance imaging (“MRI”) systemsand methods. More particularly, the invention relates to systems andmethods for tracking an interventional device that can be actuated toinduce measurable susceptibility effects.

The placement of interventional devices, such as guidewires and stents,using MRI guidance is a promising and evolving field with great clinicalpotential. One particular challenge of this field, however, has been howto develop safe and reliable methods for tracking such devices as theyare moved and manipulated within vessels or organs. The tips ofguidewires can be easily visualized using conventional x-ray fluoroscopyby applying small, radio-opaque markers to the tips. In MRI, the analogto the radio-opaque marker is a marker made of a material with asufficiently large magnetic susceptibility, relative to the surroundingtissues, such as a stainless steel tip on a nitinol wire. In MR imagesdepicting a guidewire containing such markers, a local hypointenseregion is present in the tissues adjacent to the markers, therebyresulting in a loss of clinically relevant information. Exemplary MRvisible interventional instruments of this kind are described, forexample, in U.S. Pat. Nos. 5,728,079 and 6,430,429.

The interventional instrument described in U.S. Pat. No. 5,728,079 is acatheter provided with a hollow tubular holder, and in which theindicator element includes a concentric layer of a paramagneticmaterial. The concentric paramagnetic layer is provided in the form of acylindrical sheath whose longitudinal axis is coincident with thelongitudinal axis of the holder. The paramagnetic material influencesthe magnetic resonance image of a patient to be examined by means of anMRI system. The influence the device has on MR images makes it possibleto determine the position of the interventional instrument within thebody of the patient without the instrument being directly visible.However, the influence of the paramagnetic component in the device onthe MR image adversely affects the diagnostic quality of the magneticresonance image. As a result of the influence of the indicator element,MR images will exhibit degraded or lost anatomical details in theregions adjacent to the indicator element.

The interventional instrument described in U.S. Pat. No. 6,430,429includes an indicator element for which the degree of influencing of themagnetic resonance image is adjustable, notably by rotation of theindicator element relative to the direction of the steady magnetic fieldof the MRI system. For example, the indicator element is a paramagneticstrip which may include several segments of different magneticsusceptibility. Only paramagnetic components are described in U.S. Pat.No. 6,430,429. Thus, the influence of this device on a magneticresonance image will depend on the orientation of the device relative tothe magnetic field of the MRI system. However, this adjustability of thedevice's influence is limited, and while the degree of the device'sinfluence on MR images can be reduced, it cannot be eliminated andsignal loss will still occur even with this reduced influence.

Despite being easy to locate in MR images and relatively inexpensive andsafe, the aforementioned interventional devices produce a loss of signalin the vicinity of the indicator element that, in turn, obscures thedesired region of interest: the tissue adjacent to the tip of thedevice. Thus, while the location of the tips of the aforementioneddevices can be easily identified, the nature of the tissue that thedevices are being moved through is obscured by the same effect thatallows the visualization of the devices.

It would therefore be desirable to provide an interventional device thatcan be accurately located in an MR image while also allowingvisualization of the tissue adjacent to the device by mitigating signallosses.

SUMMARY OF THE INVENTION

The present invention overcomes the aforementioned drawbacks byproviding a tracking device that can be imaged with magnetic resonanceimaging (MRI). The tracking device generally includes two or moretracking members having different magnetic susceptibilities. Thetracking device can be adjusted between a first arrangement thatproduces a local magnetic field external to the tracking device when amagnetic field, such as the magnetic field of an MRI system, is appliedto the tracking device, and a second arrangement where the localmagnetic field external to the tracking device is reduced relative tothe first arrangement. These arrangements may be referred to generallyas an “on” and “off” configuration, respectively. The tracking deviceoperates to produce a measurable local magnetic field in the “on”configuration and a reduced local magnetic field in the “off”configuration, regardless of the orientation of the tracking devicerelative to the magnetic field of the MRI system.

It is an aspect of the invention to provide a tracking device configuredto be coupled to an interventional medical device to track a position ofthe interventional medical device during an interventional medicalprocedure using an MRI system. The tracking device includes a firsttracking member having a first magnetic susceptibility and configured tobe coupled to the interventional medical device, and a second trackingmember having a second magnetic susceptibility, which is different thanthe first magnetic susceptibility, and configured to be coupled to theinterventional medical device. The first tracking member may include atleast one paramagnetic component and the second tracking member mayinclude at least one diamagnetic component. For example, the firsttracking member may include one, two, or more paramagnetic components,and the second tracking member may include one, two, or more diamagneticcomponents. At least one of the first tracking member and the secondtracking member is configured to adjust relative to the other of thefirst tracking member and the second tracking member to adjust thetracking device between a first arrangement and a second arrangement.When the first and second tracking members include more than onecomponent, these components can be independently adjusted relative toeach other to adjust the tracking device between the first and secondarrangements. The first and second magnetic susceptibilities areselected such that when the tracking device is positioned within amagnetic field of an MRI system the tracking device produces a localmagnetic field that is measurable by the MRI system when the trackingdevice is arranged in the first arrangement, and the tracking deviceproduces a local magnetic field that is reduced with respect to thelocal magnetic field produced by the tracking device in the firstarrangement when the tracking device is arranged in the secondarrangement. Thus, in the first arrangement, the tracking deviceproduces a magnetic field disturbance that is measurable with the MRIsystem, whereas in the second arrangement, magnetic field disturbancesfrom the tracking device that may be detected with the MRI system aresubstantially suppressed.

It is another aspect of the invention to provide a catheter device foruse with an MRI system. The catheter device includes a tracking devicecontaining a first annular component having a first magneticsusceptibility and a second annular component having a second magneticsusceptibility that is different than the first magnetic susceptibility.A wire is coupled to the tracking device and extends away from thetracking device through a catheter tube that is also coupled to thetracking device. The wire is configured such that when it is actuated,at least one of the first and second annular components moves relativeto the other, thereby altering local magnetic fields produced by thetracking device when the tracking device is positioned in a magneticfield of an MRI system. The catheter device may include a driver coupledto the wire for actuating the wire, and the driver may be incommunication with a processor that is configured to synchronouslyoperate the driver in response to a pulse sequence performed by the MRIsystem.

It is yet another aspect of the invention to provide a method formonitoring an interventional procedure with an MRI system and aninterventional instrument that includes a tracking device having aplurality of concentric components of opposing magnetic susceptibilitiesthat are movably engaged with each other. The method includes the stepof manipulating the tracking device so that its concentric componentsare axially aligned, thereby substantially suppressing magnetic fielddisturbances produced by the tracking device. While the interventionaldevice is positioned within a patient, the MRI system is operated toobtain a first set of images of a patient. As these images are obtained,magnetic field disturbances produced by the tracking device aresubstantially suppressed. The interventional device may be manipulatedthen to produce a measurable magnetic field disturbance by axiallydisplacing the concentric components of the tracking device relative toone another. A second set of images of the patient is then obtained withthe MRI system while the interventional device is producing themeasureable magnetic field disturbance. From the first and second set ofimages, a position of the interventional device is determined.

The foregoing and other aspects and advantages of the invention willappear from the following description. In the description, reference ismade to the accompanying drawings which form a part hereof, and in whichthere is shown by way of illustration a preferred embodiment of theinvention. Such embodiment does not necessarily represent the full scopeof the invention, however, and reference is made therefore to the claimsand herein for interpreting the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an exemplary magnetic resonance imaging(“MRI”) system for tracking a tracking device of the present invention;

FIG. 2 is a plan view of an exemplary catheter system employing atracking device in accordance with some embodiments of the presentinvention;

FIG. 3 is a perspective view of the exemplary tracking device that formsa part of the catheter device of FIG. 2;

FIG. 4 is an elevation view of a portion of the tracking device of FIG.2;

FIG. 4A is a cross-sectional view of the portion of the tracking deviceof FIG. 4 viewed along line 4A-4A;

FIG. 4B is a cross-sectional view of the tracking device of FIG. 4viewed along line 4B-4B;

FIG. 4C is a cross-sectional view of the tracking device of FIG. 4viewed along line 4C-4C;

FIG. 4D is a cross-sectional view of the tracking device of FIG. 4viewed along line 4D-4D;

FIG. 4E is a cross-sectional view of the tracking device of FIG. 4viewed along line 4E-4E;

FIG. 5A is a perspective view of paramagnetic and diamagnetic componentsof the tracking device of FIG. 3, in which a coupling of two componentsby way of a distal plate is illustrated;

FIG. 5B is a perspective view of paramagnetic and diamagnetic componentsof the catheter tracking device of FIG. 3, in which a coupling of one ofthe components to a wire by way of a proximal plate is illustrated;

FIG. 6A is a plan view of another exemplary catheter device, whichincludes a driver, employing a tracking device in accordance with someembodiments of the present invention;

FIG. 6B is a perspective view of the exemplary catheter device of FIG.6A;

FIG. 7 is a perspective view of another configuration of an exemplarytracking device that forms a part of the catheter device of FIG. 2, inwhich the tracking device includes five concentrically layeredcomponents that are partially retracted for illustrative purposes;

FIG. 7A is a cross-sectional view of the tracking device of FIG. 7viewed along line 7A-7A;

FIG. 8 is a plan view of another exemplary catheter system employing atracking device in accordance with some embodiments of the presentinvention, in which the tracking device includes a central lumen throughwhich an interventional tool or contrast agent may be passed;

FIG. 8A is a cross-sectional view of the tracking device of FIG. 8viewed along line 8A-8A;

FIG. 9A is a perspective view of a configuration of a tracking device inaccordance with some embodiments of the invention, in which the trackingdevice includes two tracking members that are composed of a plurality ofradial sectors; and

FIG. 9B is a perspective view of the tracking device of FIG. 9A in whichthe tracking device is in an “off” configuration.

DETAILED DESCRIPTION OF THE INVENTION

With initial reference to FIG. 1, a tracking device 100 for tracking aninterventional instrument during a medical procedure with a magneticresonance imaging (“MRI”) system is provided. Generally, the trackingdevice 100 includes two or more tracking members having differentmagnetic susceptibilities. As will be described in detail below, eachtracking member may include one or more components having similarmagnetic susceptibilities, with different tracking member havingdifferent magnetic susceptibilities, in general. The components mayinclude concentric cylindrical and annular components, radial sectors,annular discs, or other such shapes, as will be discussed below. Thetracking members can be adjusted between a first configuration thatproduces a local magnetic field external to the device in the presenceof an applied magnetic field, such as the magnetic field of an MRIsystem, and a second configuration where the local magnetic fieldproduced external to the device is reduced relative to the firstconfiguration. These configurations may be referred to generally as “on”and “off” configurations, respectively. The tracking device operates toproduce a measurable local magnetic field regardless of the orientationof the tracking device relative to the magnetic field of the MRI system.

Materials with positive magnetic susceptibility, such as paramagneticmaterials, become magnetized when exposed to an applied magnetic field,with the vector direction of that magnetization being in the samedirection as the applied magnetic field. In contrast, materials withnegative magnetic susceptibility, such as diamagnetic materials, becomemagnetized with the vector direction of that magnetization in theopposite direction to that of the applied magnetic field. With thetracking device 100 of the present invention, magnetic fields fromcomponents of opposing magnetic susceptibility significantly oppose eachother to reduce the overall local magnetic field produced by thetracking device when the tracking device is in an “off” configuration,thereby allowing the acquisition of magnetic resonance images withminimal signal loss from the tracking device 100. When the device isadjusted into an “on” configuration, the paramagnetic and diamagneticcomponents are moved relative to each other such that their magneticfields no longer significantly oppose each other. The result of thisrelative spacing of the components is that a larger magnetic fielddisturbance is created when the device is in the “on” position than whenthe device is in the “off” position, thereby rendering the trackingdevice visible in images acquired with an MRI system.

Referring particularly now to FIG. 1, an exemplary MRI system 200 forimaging and determining the position of the catheter tracking device 100is illustrated. The MRI system 200 includes a workstation 202 having adisplay 204 and a keyboard 206. The workstation 202 includes a processor208, such as a commercially available programmable machine running acommercially available operating system. The workstation 202 providesthe operator interface that enables scan prescriptions to be enteredinto the MRI system 200. The workstation 202 is coupled to four servers:a pulse sequence server 210; a data acquisition server 212; a dataprocessing server 214, and a data store server 216. The workstation 202and each server 210, 212, 214 and 216 are connected to communicate witheach other.

The pulse sequence server 210 functions in response to instructionsdownloaded from the workstation 202 to operate a gradient system 218 anda radiofrequency (“RF”) system 220. Gradient waveforms necessary toperform the prescribed scan are produced and applied to the gradientsystem 218, which excites gradient coils in an assembly 222 to producethe magnetic field gradients G_(x), G_(y), and G_(z) used for positionencoding MR signals. The gradient coil assembly 222 forms part of amagnet assembly 224 that includes a polarizing magnet 226 and awhole-body RF coil 228.

RF excitation waveforms are applied to the RF coil 228, or a separatelocal coil (not shown in FIG. 1), by the RF system 220 to perform theprescribed magnetic resonance pulse sequence. Responsive MR signalsdetected by the RF coil 228, or a separate local coil (not shown in FIG.1), are received by the RF system 220, amplified, demodulated, filtered,and digitized under direction of commands produced by the pulse sequenceserver 210. The RF system 220 includes an RF transmitter for producing awide variety of RF pulses used in MR pulse sequences. The RF transmitteris responsive to the scan prescription and direction from the pulsesequence server 210 to produce RF pulses of the desired frequency,phase, and pulse amplitude waveform. The generated RF pulses may beapplied to the whole body RF coil 228 or to one or more local coils orcoil arrays (not shown in FIG. 1).

The RF system 220 also includes one or more RF receiver channels. EachRF receiver channel includes an RF amplifier that amplifies the MRsignal received by the coil 228 to which it is connected, and a detectorthat detects and digitizes the I and Q quadrature components of thereceived MR signal. The magnitude of the received MR signal may thus bedetermined at any sampled point by the square root of the sum of thesquares of the I and Q components:M=√{square root over (I ² +Q ²)}  (1);

and the phase of the received MR signal may also be determined:

$\begin{matrix}{\phi = {{\tan^{- 1}\left( \frac{Q}{I} \right)}.}} & (2)\end{matrix}$

The pulse sequence server 210 also optionally receives patient data froma physiological acquisition controller 230. The controller 230 receivessignals from a number of different sensors connected to the patient,such as electrocardiograph (“ECG”) signals from electrodes, orrespiratory signals from a bellows or other respiratory monitoringdevice. Such signals are typically used by the pulse sequence server 210to synchronize, or “gate,” the performance of the scan with thesubject's heart beat or respiration.

The pulse sequence server 210 also connects to a scan room interfacecircuit 232 that receives signals from various sensors associated withthe condition of the patient and the magnet system. It is also throughthe scan room interface circuit 232 that a patient positioning system234 receives commands to move the patient to desired positions duringthe scan. The scan room interface circuit 232 may also communicate withthe tracking device 100 to direct manipulate the tracking device 100between its “on” and “off” configurations. For example, the trackingdevice 100 can be programmed to operate synchronously with therepetition time (“TR”) of a pulse sequence, thereby providing dataacquisition of the tracking device 100 in both its “on” and “off”configurations.

The digitized MR signal samples produced by the RF system 220 arereceived by the data acquisition server 212. The data acquisition server212 operates in response to instructions downloaded from the workstation202 to receive the real-time MR data and provide buffer storage, suchthat no data is lost by data overrun. In some scans, the dataacquisition server 212 does little more than pass the acquired MR datato the data processor server 214. However, in scans that requireinformation derived from acquired MR data to control the furtherperformance of the scan, the data acquisition server 212 is programmedto produce such information and convey it to the pulse sequence server210. For example, during prescans, MR data is acquired and used tocalibrate the pulse sequence performed by the pulse sequence server 210.Also, navigator signals may be acquired during a scan and used to adjustthe operating parameters of the RF system 220 or the gradient system218, or to control the view order in which k-space is sampled. The dataacquisition server 212 may also be employed to process MR signals usedto detect the arrival of contrast agent in a magnetic resonanceangiography (“MRA”) scan. In all these examples, the data acquisitionserver 212 acquires MR data and processes it in real-time to produceinformation that is used to control the scan.

The data processing server 214 receives MR data from the dataacquisition server 212 and processes it in accordance with instructionsdownloaded from the workstation 202. Such processing may include, forexample: Fourier transformation of raw k-space MR data to produce two orthree-dimensional images; the application of filters to a reconstructedimage; the performance of a backprojection image reconstruction ofacquired MR data; the generation of functional MR images; and thecalculation of motion or flow images.

Images reconstructed by the data processing server 214 are conveyed backto the workstation 202 where they are stored. Real-time images arestored in a data base memory cache (not shown in FIG. 1), from whichthey may be output to operator display 212 or a display 236 that islocated near the magnet assembly 224 for use by attending physicians.Batch mode images or selected real time images are stored in a hostdatabase on disc storage 238. When such images have been reconstructedand transferred to storage, the data processing server 214 notifies thedata store server 216 on the workstation 202. The workstation 202 may beused by an operator to archive the images, produce films, or send theimages via a network to other facilities.

Because the influence on the magnetic resonance images by the trackingdevice 100 is adjustable by relative movement of the tracking members ofthe tracking device 100, the tracking members including paramagnetic anddiamagnetic components, MR images that are influenced by the trackingdevice 100 may be acquired. The influence of the tracking device 100 onthese images can be visualized and measured, such that a location of thetracking device 100 can be accurately determined. Furthermore, becausethe tracking device 100 can be switched between an “on” configuration,in which the tracking device 100 creates significant susceptibilityartifacts allowing the location of the tracking device 100 to bedeterminable, but in which signals from adjacent tissues are distorted,and an “off” configuration, in which the tracking device 100 is notvisible, but in which signals from adjacent tissues are minimallydistorted, clinically useful images of a patient can be obtained whilethe tracking device 100 is positioned within a patient. For example, byadjusting the relative arrangement of the tracking members to adjust thetracking device between a second and first arrangement, the influence ofthe tracking device 100 on the MR images is reduced so that signallosses in tissues adjacent to the tracking device 100 due to magneticsusceptibility artifacts from the tracking device 100 are substantiallysuppressed. It is noted that each tracking member may include aplurality of components and that each of these components may beadjusted individually or in combination when the tracking device isadjusted between the first and second arrangement.

The tracking device 100 of the present invention generally operates byadjusting the arrangement of the tracking device by adjusting one of afirst tracking member having a first magnetic susceptibility and asecond tracking member having a second magnetic susceptibility relativeto each other. Because of the disparate magnetic susceptibilities ofthese first and second tracking members, the local magnetic fieldsproduced by the tracking device 100 in the presence of an appliedmagnetic field can be varied from a measurable state when the first andsecond tracking members are in a first arrangement, to a minimal statewhen the first and second tracking members are in a second arrangement.In the minimal state, the local magnetic fields produced by the firstand second tracking members substantially cancel each other out, whereasin the measurable state the local magnetic fields produced by the firstand second tracking members do not cancel each other out, therebyproducing a magnetic field disturbance that is measurable with an MRIsystem. Generally, this second arrangement is referred to as an “off”configuration and the first arrangement is referred to as an “on”configuration. When the local magnetic fields produced by the trackingdevice 100 in response to an applied magnetic field are significantlynonzero, such as when the tracking device 100 is in the “on”configuration, the position of the tracking device 100 may be measuredor identified by an MRI system in vivo.

An exemplary configuration of the tracking device 100 as it is coupledto a catheter system is illustrated in FIGS. 2 and 3. While thisexemplary configuration is described with respect to the tracking of acatheter system, it will be appreciated by those skilled in the art thatthe tracking device 100 may similarly be coupled to other interventionalinstruments such as biopsy needles and ablation probes, such as thosefor thermal ablation and cryoablation. The tracking device 100 includesa cylindrical wire 102 about which a first annular sheath 104 is movablydisposed. A second annular sheath 106 is movably disposed about thefirst annular sheath 104. A distal plate 108 may be coupled at one endof the cylindrical wire 102 to the cylindrical wire 102 and the secondannular sheath 106, and a proximal plate 110 may be coupled at the otherend of the cylindrical wire 102 to the first annular sheath 104.

The cylindrical wire 102 and the second annular sheath 106 are composedof a paramagnetic material, such as titanium; however, it will beappreciated by those skilled in the art that other suitable paramagneticmaterials may also be used. The cylindrical wire 102 and second annularsheath 106 are preferably composed of materials having the same magneticsusceptibility; however, in some configurations the magneticsusceptibilities of the cylindrical wire 102 and second catheter sheath106 may differ. The first annular sheath 104 is composed of adiamagnetic material, such as graphite; however, it will be appreciatedby those skilled in the art that other suitable diamagnetic materialsmay also be used, such as bismuth. In general, it is preferable that theparamagnetic material have a substantially equal but opposite magneticsusceptibility as the diamagnetic material; however, this condition isnot required for suitable operation of the tracking device 100. Thecylindrical wire 102, first annular sheath 104, and second annularsheath 106 may be dimensioned such that they are flexible, which is anamenable attribute for use in a catheter.

Thus, in general, the tracking device 100 includes a first trackingmember composed of paramagnetic materials and a second tracking membercomposed of diamagnetic materials. For the tracking device 100configuration illustrated in FIGS. 3 and 4, the first tracking member iscomposed of the cylindrical wire 102 and second annular sheath 106,whereas the second tracking member is composed of the first annularsheath 104. It should be appreciated by those skilled in the art thatthe tracking device 100, which includes components composed ofparamagnetic and diamagnetic materials, is visible with x-ray imaging.Thus, the tracking device may also be beneficial for interventionalprocedures that require or benefit from x-ray imaging, such as x-rayfluoroscopy and x-ray computed tomography.

The cylindrical wire 102, first annular sheath 104, and second annularsheath 106 are contained within a housing 112 that is composed, forexample, of a biocompatible material such as a biocompatible polymer ormedical grade polyvinyl chloride (“PVC”). The housing 112 forms astructure designed to hold the cylindrical wire 102, first annularsheath 104, and second annular sheath 106 in place while allowing theseelements to move relative to each other along a common longitudinal axis113. The housing 112 is also configured to provide a sterile operatingenvironment for the tracking device components.

A catheter tube 114 extends from its distal end 116 proximally towardsits proximal end 118 and is coupled at its distal end 116 to the housing112 of the tracking device 100. The tracking device 100 is coupled to awire 120 that extends away from the tracking device 100 through a lumen122 in the housing 112 towards the proximal end 118 of the catheter tube114. The wire 120 may be composed of a material having a magneticsusceptibility that is similar to tissue, such as copper or a polymer. Atip 124 is formed at the distal end 126 of the housing 112. The tip 124is preferentially rounded so as to not damage vessel walls when in use;however, the tip 124 may also be shaped otherwise.

As noted, in some configurations, the tracking device 100 may bedimensioned so that it can be coupled to a catheter. For example, thetracking device 100 is dimensioned to be two French or larger, dependingon the particular clinical application; however, with suitablemanufacturing the tracking device may be dimensioned to be even smallerthan two French.

Referring now to FIGS. 4 and 4A, the cylindrical wire 102, first annularsheath 104, and second annular sheath 106 have substantially similarlengths, and their relative thicknesses are dimensioned so that when thetracking device 100 is positioned within a magnetic field, such as themain magnetic field of an MRI system, the local magnetic fields producedby the respective components substantially cancel each other out,thereby resulting in the tracking device 100 having a substantiallysuppressed magnetic field disturbance.

The distal plate 108 may include a central aperture 109 so that thedistal plate 108 may be coupled to the cylindrical wire 102 and thesecond annular sheath 106. In such an instance, the distal plate 108 maybe dimensioned so that the outer diameter of the distal plate 108 issmaller than the outer diameter of the second annular sheath 106 toensure a strong joint between the two components. The outer diameter ofthe distal plate may also be larger than the outer diameter of the firstannular sheath 104 so that the distal plate 108 is not errantly coupledto the first annular sheath 104. To further ensure that the distal plate108 is not errantly coupled to the first annular sheath 104, the innerdiameter of the distal plate 108, that is the diameter of the centralaperture 109, may be smaller than the inner diameter of the firstannular sheath 104.

The proximal plate 110 may be dimensioned so that the outer diameter ofthe proximal plate 110 is smaller than the outer diameter of the firstannular sheath 104 so that a strong joint between the two components canbe formed. The distal and proximal plates 108, 110 may be composed of amaterial such as copper; however, a polymer material may also be used.The distal plate 108 may be coupled to the cylindrical wire 102 andsecond annular sheath 106 by way of an epoxy; however, other attachmentmeans such as brazing may also be employed. The proximal plate 110 maybe similarly coupled to the first annular sheath 104.

The tracking device 100 may be manipulated between the “on” and “off”configurations by way of a handle 136 that is coupled to the wire 120 atthe proximal end of the wire 120. By manipulating the handle 136, thewire 120 may be translated axially along the longitudinal axis 113 ofthe tracking device 100. This translation of the wire 120 results in thefirst tracking member and the second tracking member being adjustedbetween the two arrangements of the “on” and “off” configurations of thetracking device 100. For example, translation of the wire 120 may resultin the first annular sheath 104 being moved relative to the cylindricalwire 102 and second annular sheath 106, while in other configurations ofthe tracking device, translation of the wire 120 may result in one orboth of the cylindrical wire 102 and second annular sheath 106 beingmoved relative to the first annular sheath 104.

In general, it will be appreciated by those skilled in the art that thedesired effect of the tracking device 100 may be achieved by moving oneor more of the tracking components relative to the others. Preferably,the components will be moved such that all of the diamagnetic componentsand all of the paramagnetic components are displaced relative to eachother. Such an arrangement provides the maximal effect; however, ameasurable alteration of the magnetic field of an MRI can be achievedwith the displacement of only one component relative to the others.

It is preferable that the magnetically susceptible components beconfigured to retract proximally away from the distal end 126 of thetracking device 100 so that the components remain within the housing112. This configuration allows for the inner components in the trackingdevice 100 to be kept separate from the patient, thereby maintainingsterility of these components and avoiding any toxicity that may occurby exposure of the body to the materials used for the inner componentsof the tracking device 100. It should be appreciated by those skilled inthe art, however, that the tracking device 100 can also be configuredsuch that the magnetically susceptible components be translated distallyinto an enclosed void at the tip of the housing 112.

In some configurations the wire 120 may be composed of a paramagneticmaterial, or contain a paramagnetic tip portion, and may replace thecylindrical wire 102. In such configurations, the first annular sheath104 may be coupled to the interior of the housing 112 at the distal endof the first annular sheath 104. Thus, in such configurations the wire120 and second annular sheath 106 may be coupled together such thattranslation of the wire 120 results in translation of the second annularsheath 106 as well. For example, the wire 120 may be coupled to thesecond annular sheath 106 by way of a washer that is, for example,brazed to the wire 120 and second annular sheath 106.

Referring briefly to FIGS. 4B-4E, in cross-section the components of thetracking device 100, including the housing 112; optional distal andproximal plates 108, 100; cylindrical wire 102; first and second annularsheaths 104, 106; and wire 120 are all substantially concentric andaligned along a common longitudinal axis.

Referring briefly to FIG. 5A, the cylindrical wire 102 and secondannular sheath 106 may be coupled together by way of the distal plate108. The distal plate 108 may be shaped like an annular disc so that theedges of the distal plate 108 may be coupled to both the cylindricalwire 102 and second annular sheath 106 by way of brazing.

Referring briefly now to FIG. 5B, the first annular sheath 104 may becoupled to the wire 120 by way of the proximal plate 110. When the firstannular sheath 104 is composed of a non-metallic material, the firstannular sheath 104 and wire 120 may be coupled to the proximal plate 110using an epoxy. However, when the first annular sheath 104 is composedof a metallic material, then brazing can be used to couple the firstannular sheath 104 and wire 120 to the proximal plate 110.

Another exemplary configuration of the provided tracking device 100 asit is coupled to a catheter system is illustrated in FIGS. 6A and 6B.The tracking device 100 may be coupled to a catheter tube 114 that iscoupled to a catheter connector 128. The wire 120 may extend through thecatheter tube 114 and catheter connector 128, with its proximal end 129coupled to a wire holder 130. The wire holder may be coupled to a driver132, such as a piezoelectric motor. MRI compatible piezoelectric motorsare known in the art. For example, an M-683 PILine precision microtranslation stage piezomotor (Physik Instrumente, GmbH & Co. KG;Karlsruhe, Germany) may be used and programmed to translate the wire 120by a few millimeters with a 0.1 micrometer resolution. The catheterconnector is coupled to a driver holder 134, which holds the driver 132in place during operation. The driver holder 134 may be composed of, forexample, PVC. Similar to the configuration of the tracking device 100illustrated in FIG. 2, the tracking device 100 may be manipulatedbetween its “on” and “off” configurations by way of the driver 132,which can be operated to translate the wire 120.

While the tracking device 100 configuration illustrated in FIGS. 4 and4A include three magnetically susceptible components (102, 104, 106), itwill be appreciated by those skilled in the art that additionalconcentrically layered components can be incorporated into the trackingdevice 100 to provide different local magnetic field configurationsproduced by the tracking device 100 in a magnetic field. For example,and referring now to FIGS. 7 and 7A, the tracking device 100 can includemore than three, such as five, magnetically susceptible components. Thetracking device 100 may include a cylindrical wire 152 about which afirst annular sheath 154 is movably disposed. This first annular sheath154 can be movably positioned within a second annular sheath 156, whichin turn may be positioned within a third annular sheath 158, which inturn may be positioned within a fourth annular sheath. In general, anynumber of concentrically layered components of alternating magneticsusceptibilities may be used.

While it is preferable that an odd number of components be used with thecentral component being composed of a paramagnetic material, it will beappreciated by those skilled in the art that other arrangements arepossible. For example, an even number of components may be used, such astwo components, and components having like or similar magneticsusceptibilities may be adjacent to each other. Moreover, the centralcomponent need not be composed of a paramagnetic material, but may becomposed of a diamagnetic, or magnetically neutral, material. For theconfiguration of the tracking device 100 illustrated in FIGS. 7A and 7B,the first tracking member is composed of the cylindrical wire 152, thesecond annular sheath 156, and the fourth annular sheath 160, whereasthe second tracking member is composed of the first annular sheath 154and third annular sheath 158. Thus, generally, the cylindrical wire 152,the second annular sheath 156, and the fourth annular sheath 160 arecomposed of a paramagnetic material, whereas the first annular sheath154 and third annular sheath 158 are composed of a diamagnetic material.In some configurations, it may be beneficial to move individualcomponents within a tracking member relative to each other. For example,the cylindrical wire 152, second annular sheath 156, and fourth annularsheath 160 may be configured so that they can be moved individuallyrelative to each other instead of in unison. When more than threeconcentric components are present in the tracking device 100, it will beappreciated by those skilled in the art that a distal plate 108 andproximal plate 110 may similarly be used to couple components havingsimilar or dissimilar magnetic susceptibilities.

Referring now to FIGS. 8 and 8A, in some configurations the trackingdevice 100 includes a central lumen 138 extending from the tip 124 ofthe tracking device proximally through the tracking device 100. Forexample, the central lumen 138 extends through the housing 112, thedistal plate 108, the cylindrical wire 102, the proximal plate 110, thewire 120, and the handle 136. The central lumen 138 is dimensioned sothat an interventional tool 140 can be provided through the centrallumen 138. Exemplary interventional tools 140 include catheterguidewires, ablation probes, balloon catheters, stents, or needles. Aliquid contrast agent, such as gadolinium-DTPA or a radioopaque dye, mayalso be provided through the central lumen 140 to aid in visualizationof the tissue and microvasculature beyond the tip 124 of the trackingdevice 100.

The materials and dimensions of the magnetically susceptible componentsare selected by, for example, first measuring the relative magneticsusceptibilities of the paramagnetic and diamagnetic materials using,for example, an Evans balance. The readings from an Evans balance are,for example, 953+/−1 percent for titanium and −745+/−1 percent forgraphite. These measured parameters may then be used in an optimizationprocedure to design the component thicknesses that minimize the magneticfields outside of the tracking device 100 when the ends of thecomponents are aligned.

A magnetostatic simulation may be used, with the magnetic fieldsurrounding the components computed for each iteration of theoptimization. A cost function that is the sum of the absolute values ofthe magnetic field offsets at points within a region-of-interestadjacent to the tracking device 100 is used during this optimization. Anexemplary region-of-interest may be 4.1 millimeters by 2 millimeters insize. The diameter of the outer paramagnetic layer may be fixed to adesired size, such as 3 millimeters, where then the free parameters tobe optimized are the outer diameters of other components. The costfunction is systematically computed over a range, and a minimum in thecost function is found. For the three-layer configuration of thetracking device 100, such as the one illustrated in FIG. 4, an exemplaryset of outer diameters is as follows: 3 millimeters for the secondannular sheath 106, 2.43 millimeters for the first annular sheath 104,and 1.11 millimeters for the cylindrical wire 102. These diameters arespecific to the measured magnetic properties noted above, but it will beappreciated by those skilled in the art that the same optimizationprocedure may be used with other materials, yielding differentdimensions, and that the optimization may be extended to devices withmore than three layers.

As described above, the first and second tracking members that form apart of the tracking device 100 may include a plurality of concentriccylindrical or annular components arranged about a common axis. In otherconfigurations, however, the first and second tracking members mayinclude components shaped and arranged in different configurations. Forexample, referring to FIGS. 9A and 9B, an alternative configuration of afirst and second tracking member is illustrated. Such tracking membersare formed as a plurality of radial sectors 172 arranged to form aplurality of annular discs 174. The annular discs 174 are aligned alonga longitudinal axis 176 to form an annular structure 178. Preferably,each annular disc 174 includes at least one radial sector 172 associatedwith the first tracking member and at least one other radial sector 172associated with the second tracking member. Generally, the firsttracking member is composed of those radial sectors 172 having a firstmagnetic susceptibility, and the second tracking member is composed ofthose radial sectors 172 having a second magnetic susceptibility that isdifferent that the first magnetic susceptibility. A subset of theannular discs 174 may be rotated about the longitudinal axis 176relative to the others, so that the radial sectors 172 having similarmagnetic susceptibilities move through a first arrangement, such as theone illustrated in FIG. 9A, and a second arrangement, such as the oneillustrated in FIG. 9B. The first arrangement illustrated in FIG. 9A isan “off” configuration, whereas the second arrangement illustrated inFIG. 9B is an “on” position. The annular discs 174 may be rotated, forexample, by the wire 120 which may be configured to engage each of theannular rings 174 while extending through an aperture 180 in the annularstructure 178. In such a configuration, it may be beneficial for thewire 120 to be composed of a material having magnetic susceptibilitysubstantially similar to tissue.

Having described the general structure of the tracking device 100, andvarious exemplary configurations thereof, a description of a generaloperation of the tracking device 100 is now provided. By way of example,the tracking device 100 may be used during the crossing of an occludedblood vessel using an interventional instrument that incorporates thetracking device 100 at the tip of the instrument. Magnetic resonanceimages may be acquired during the process of pushing the instrumentthrough the proximal side of the occlusion, along the track of theoccluded vessel, and out of the distal side of the occlusion. Byalternately acquiring images with the tracking device 100 in its “on”and “off” configurations, the position of the tracking device 100 andinstrument can be accurately determined while magnetic resonance images,depicting the tissue immediately adjacent to the instrument tip can beacquired with substantially no distortion from susceptibility artifacts.Accurate tracking device 100 location measurements can be indicated onthe acquired images of the section of occluded vessel that is in theimmediate path of the tracking device 100. This gives the clinicianconfidence that the device is on course while the next few millimetersof occlusion are crossed, after which a new location measurement andcorresponding, minimally distorted anatomical image of the occludedvessel are acquired.

To quickly and accurately measure the location of the tracking device100, several different types of magnetic resonance images may beobtained. For example, by employing the so-called “white marker”phenomenon described in published U.S. patent application Ser. No.11/257,415, images can be acquired in which signal is received only fromthe immediate vicinity of the tracking device 100, with greatlysuppressed signal from all other locations. The position of the trackingdevice 100 can thus be quickly estimated from projection images havingcoarse spatial resolution. Then, a measurement with higher spatialresolution can be performed, limiting the field-of-view to the volumeestimated from the projection images. This second acquisition can beperformed with the white-marker imaging mentioned above, withconventional gradient-echo imaging, or with any suitable magneticresonance imaging method that results in images influenced by the devicein the “on” configuration.

In some embodiments, a two-step process is used to determine thelocation of the tracking device 100. In the first step, an approximatelocation of the tracking device 100 is computed from projection images.With the device in the “on” configuration, where substantially maximalmagnetic field disturbance is produced, the aforementioned white-markertechnique may be used to determine the location of the tracking devicewith an accuracy of approximately plus-or-minus 0.5 centimeters. Such amethod includes computing the location of signal maxima in imagesacquired with the white marker method. In the second step, atwo-dimensional slice location is positioned at the location computed inthe preceding step, and a two-dimensional image is acquired using, forexample, a slice-selective gradient echo pulse sequence. The resultingimage is then input into a fitting algorithm described below. These twostages can be alternated rapidly in order to continuously update theposition displayed to the operator on the console, or physician in theoperating room.

To accurately measure the position of the tracking device 100, theimaging data can be fitted numerically to a model of the effect of thetracking device 100 on a magnetic resonance image. Parameters relatingto the configuration of the paramagnetic and diamagnetic componentsrelative to one another will be known and do not need to be derived byfitting. Thus, the parameters to be fit are the spatial coordinates ofthe tracking device 100 as well as the orientation of the trackingdevice 100 relative to the main magnetic field of the MRI system.

Two images are used in the numerical fit: one depicting the trackingdevice 100 in the “on” configuration, and one depicting the trackingdevice 100 in the “off” configuration. Preferably, these images arereconstructed from image data that was acquired as close in time as isfeasible. The phase difference between these two images is calculatedand used as input to the fitting process.

The numerical model fitting generally proceeds as follows. Theapproximate tip position and tracking device angle relative to the mainmagnetic field are input as the starting point in the fit. A simulationof the MRI scanning process (excitation, application of gradients, datasampling) is used to compute the images that would result from thedevice, in both the “on” and “off” configurations, within a uniformmedium. The phase difference between these two simulated images iscomputed. A cost function is then computed for the current, or initial,position of the tracking device 100. In some embodiments, this costfunction includes the sum-of-squares of the difference between the realand simulated phase-difference images. The process is then iterated byan optimization solver, with the cost function computed for an array ofpositions until a minimum in the cost function is found. The positionthat corresponds to this minimum is identified and stored as thecomputed position.

For accurate computation of the position of the device tip, it is usefulto have information about the orientation of the device relative to themain magnetic field of the MRI system. When the tracking device 100 isin the on position, two regions of signal disturbance are generated: oneat the distal end of the tracking device, and one at the proximal end ofthe tracking device. These regions of signal disturbance appear inimages produced with the MRI system, and the angle between thesedisturbances can be used as an estimate of the angle of the devicerelative to the main magnetic field. This orientation information canthen be used as an input to a fitting algorithm used to compute theposition of the tracking device 100.

To enable the acquisition of images with the tracking device 100 in boththe “on” and “off” configurations in rapid succession, an actuator, suchas the driver 132 described above, can be used to rapidly and accuratelyactuate the tracking device 100 between the “on” and “off”configurations. In some embodiments, this actuator may be synchronizedwith the MRI system such that the tracking device 100 is actuated to the“off” configuration from the “on” configuration in a minimal timeinterval, such as twenty milliseconds, after the acquisition of thefirst image. This synchronization can be accomplished by triggering theactuator by a transistor-transistor logic (“TTL”) pulse that is providedby the MRI system at the start of each iteration of a pulse sequence. Asdescribed above, however, a driver 132 need not be used to manipulatethe tracking device 100; rather, manual manipulation of the trackingdevice 100 can similarly be implemented.

By alternately acquiring images with the paramagnetic and diamagneticcomponents of the tracking device 100 in their “on” and “off” positions,the location of the tracking device 100 can be accurately determined,while magnetic resonance images, showing the tissue immediately in frontof and surrounding the tracking device can be acquired with minimaldistortion. These accurate tracking device location measurements can beoptionally overlaid on the MR image of the region-of-interest, such asan occluded vessel, that is in the immediate path of the device. Thisprovides the clinician confidence that the device is on course for thenext few millimeters of occlusion crossing, after which a newtip-location measurement and corresponding, minimally distorted,anatomical image of the occluded vessel may be acquired.

The present invention has been described in terms of one or morepreferred embodiments, and it should be appreciated that manyequivalents, alternatives, variations, and modifications, aside fromthose expressly stated, are possible and within the scope of theinvention.

The invention claimed is:
 1. A tracking device configured to be coupledto an interventional medical device to track a position of theinterventional medical device during an interventional medical procedureusing a magnetic resonance imaging (MRI) system, the tracking devicecomprising: a first tracking member having a first magneticsusceptibility and configured to be coupled to an interventional medicaldevice; a second tracking member having a second magnetic susceptibilitydifferent than the first magnetic susceptibility and configured to becoupled to the interventional medical device; wherein the first trackingmember and the second tracking member are configured to adjust relativeto each other to adjust the tracking device between a first arrangementand a second arrangement; and wherein the first magnetic susceptibilityand the second magnetic susceptibility are selected such that when thetracking device is positioned within a magnetic field of an MRI system:when the tracking device is arranged in the first arrangement thetracking device produces a local magnetic field that is measurable bythe MRI system; and when the tracking device is arranged in the secondarrangement the tracking device produces a local magnetic field that isreduced with respect to the local magnetic field produced by thetracking device in the first arrangement.
 2. The tracking device asrecited in claim 1 further comprising a wire coupled to at least one ofthe first tracking member and the second tracking member for adjustingthe tracking device between the first arrangement and the secondarrangement.
 3. The tracking device as recited in claim 1 furthercomprising: a driver; a wire extending along an axis from a distal endto a proximal end, the distal end being coupled to the at least one ofthe first tracking member and the second tracking member, and theproximal end being coupled to the driver; and wherein the driver isconfigured to actuate the wire such that one of the first trackingmember and the second tracking member is moved relative to the other,thereby adjusting the tracking device between the first arrangement andthe second arrangement.
 4. The tracking device as recited in claim 1 inwhich the first tracking member comprises a plurality of paramagneticradial sectors and the second tracking member comprises a plurality ofdiamagnetic radial sectors, the paramagnetic radial sectors anddiamagnetic radial sectors being configured to rotate about an axis ofrotation.
 5. The tracking device as recited in claim 1 in which thefirst tracking member comprises at least one paramagnetic component andthe second tracking member comprises at least one diamagnetic component.6. The tracking device as recited in claim 5 in which the at least oneparamagnetic component and the at least one diamagnetic component areconcentrically arranged about an axis.
 7. The tracking device as recitedin claim 6 in which the at least one paramagnetic component and the atleast one diamagnetic component are arranged about the axis inalternating layers of paramagnetic and diamagnetic components.
 8. Thetracking device as recited in claim 6 in which: the at least oneparamagnetic component and the at least one diamagnetic component havesubstantially similar lengths; ends of the at least one paramagneticcomponent and the at least one diamagnetic component are substantiallyaligned when the tracking device is arranged in the second arrangement;and ends of the at least one paramagnetic component and the at least onediamagnetic component are not aligned when the tracking device isarranged in the first arrangement.
 9. The tracking device as recited inclaim 6 in which the at least one paramagnetic component comprises threeconcentric paramagnetic components, and the at least one diamagneticcomponent comprises two diamagnetic components.
 10. The tracking deviceas recited in claim 8 in which each of the three concentric paramagneticcomponents and each of the two diamagnetic components are configured tomove independently of the others when the first tracking member and thesecond tracking member are adjusted relative to each other.
 11. Thetracking device as recited in claim 5 in which the at least oneparamagnetic component includes a paramagnetic wire extending along anaxis from a proximal end to a distal end and a paramagnetic sheathcoupled to the paramagnetic wire and extending along the axis from aproximal end to a distal end.
 12. The tracking device as recited inclaim 11 in which the at least one diamagnetic component includes adiamagnetic sheath extending along the axis from a proximal end to adistal end, the diamagnetic sheath being movably positioned between theparamagnetic wire and the paramagnetic sheath.
 13. The tracking deviceas recited in claim 12 further comprising a plate coupled to thediamagnetic sheath and structured to engage a wire that when actuatedaxially displaces the diamagnetic sheath along the axis of thediamagnetic sheath.
 14. The tracking device as recited in claim 12further comprising an annular plate structured to couple theparamagnetic wire to the paramagnetic sheath.
 15. The tracking device asrecited in claim 14 further comprising a wire coupled to the annularplate, the wire being configured to axially displace the paramagneticwire and paramagnetic sheath when actuated.
 16. A catheter device foruse with a magnetic resonance imaging (MRI) system, the catheter devicecomprising: a tracking device comprising: a first annular componenthaving a first magnetic susceptibility; a second annular componenthaving a second magnetic susceptibility different than the firstmagnetic susceptibility; a catheter coupled to the tracking device; awire coupled to the tracking device and extending along an axis from aproximal end to a distal end through a lumen in the catheter; whereinthe first annular component and the second annular component are movedrelative to each other when the wire is actuated, thereby altering localmagnetic fields produced by the tracking device when the tracking deviceis positioned in a magnetic field of an MRI system.
 17. The catheterdevice as recited in claim 16 in which the first and second annularcomponents have substantially similar lengths, and in which the localmagnetic fields produced by the tracking device produce a magnetic fielddisturbance measurable by the MRI system when ends of the first andsecond annular components are not aligned.
 18. The catheter device asrecited in claim 16 in which the first annular component is movablypositioned within the second annular component, and in which the wireextends through the first annular component and is coupled to the secondannular component at a distal end of the second annular component. 19.The catheter device as recited in claim 16 in which the tracking devicefurther comprises a cylindrical component movably positioned within thefirst annular component, the cylindrical component having a similarmagnetic susceptibility as the second annular component; and in whichthe wire is coupled to at least one of the cylindrical component and thefirst annular component.
 20. The catheter device as recited in claim 16in which the wire includes a lumen extending from the proximal end tothe distal end of the wire, the lumen being dimensioned to receive atleast one of an interventional tool and a contrast agent.